My 21 had foam on the outside of the stringers, under the rod boxes as well...and like you, I didn't want water in there...so here's what I did...
And after installation, ran the hoses back to the bilge...bearing dust caps, cheap thru-hulls, hose and hose clamps...I could leave the plug out and run water from a garden hose through there...lol...worked like a charm...
HEY RB...have ya found this plumbin' job yet??